Charged Particle Beam Irradiation System

ABSTRACT

It is to prevent an image drift from occurring caused by a specimen being charged when observing the specimen including an insulating material. 
     A first scan is performed in a predetermined direction on scanning line and in a predetermined sequential direction of scanning lines and a second scan is performed in a scanning direction different from the predetermined scanning direction and in a sequential direction different from the predetermined sequential direction. An image may be created by repeating the process of executing the second scan after executing the first scan and by requiring the arithmetic average of the frames obtained by the second scans. An image may be created by averaging arithmetically at least one frame obtained by the first scan and at least one frame obtained by the second scan.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/182,709 filed Jul. 30, 2008 and claims priority of Japanese patentapplication no. 2007-199933 filed Jul 31, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a charged particle beam irradiation systemwhich irradiates a specimen with charged particles such as electron, andparticularly to a charged particle beam irradiation system whichanalyzes the aspect on the surface of a specimen using a secondarydischarged signal.

2. Description of the Related Art

In the semiconductor manufacturing industry, a scanning electronmicroscope (SEM), a CD-SEM for performing critical dimension scanning,and an inspection SEM for inspecting the pattern shape and the like arewidely used as an inspection device of a semiconductor device.

In these days, a highly integrated semiconductor device has beendeveloped rapidly in the semiconductor manufacturing industry. Accordingto this, various kinds of electrically-insulating thin films are formedon a semiconductor device. When electron beam is irradiated on aninsulating material, electric charges are accumulated through secondaryelectron discharge and a specimen becomes charged. When an electricfield is produced by the charge of the specimen, the electron beamincident on the specimen is bent and the irradiation position of theelectron beam is deflected.

The amount of electric charges on the specimen is determined by theamount of an incident electron beam, the physical property of aninsulating material, the presence or absence of a path having diffusionon electric charges and its resistance, and the like.

In the electron beam scan, a scanning area usually rectangular isscanned sequentially from one end to the other. The accumulated electriccharges within the scanning area are spatially asymmetrical. Theelectric charges are diffused through a conductive material existingwithin the scanning area and its vicinity. The accumulated electriccharges are accompanied by a time fluctuation of a large time constant.According to the spatial asymmetry and the temporal fluctuation of theaccumulated electric charges, the electric field spatially andtemporally fluctuates. According to this, the irradiation position ofthe electron beam fluctuates spatially and temporally.

In the case of a scanning electron microscope, an image is shiftedaccording to the charge of the specimen. Hereinafter, this phenomenon isreferred to as “image drift”. Generally, in the scanning electronmicroscope, the same area is scanned a plurality of times (each scan isreferred to as “frame”) and the obtained images are averaged, to improvethe S/N ratio. Therefore when the positions of the images are deflectedfrom each other in the respective frames, the image obtained as theaveraging result deteriorates remarkably in resolution. A chargedparticle beam irradiation system which irradiates a charged particlebeam also has the same problem.

Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No.2005-142038,

Non-Patent Document 1: “Charge Control During Photomask CriticalDimension (CD) Metrology” written by David Joy, International SEMATECTechnology Transfer #3114452B-ENG(2004),

Non-Patent Document 2: John C. Russ: Computer-Assisted Microscopy;Plenum Publishing Corp. New York 1990: pp 40-41

SUMMARY OF THE INVENTION

Various methods for preventing a specimen from being charged have beenhitherto known. For example, there is a method of depositing an electricconductor such as metal on a specimen when observing the specimen. Acoating device and the like are provided for practical use in order toperform this method. This method for analyzing a specimen, however, maybe a destructive inspection and it cannot be adopted to a productinspection in a semiconductor industry.

In the method disclosed in Japanese Patent Application Laid-Open (JP-A)No. 2005-142038, the scanning order in the scanning area is changed.This method is on the assumption that the electric charges accumulatedin the respective scans are diffused at the same time constant as thescanning periodic cycle. This method, however, is defective in efficacywhen the diffusion constant of the electric charges becomes somehundreds times longer than the scanning cycle. This happens, forexample, when observing a liquid crystal optical element and aphotomask.

In the method disclosed in the “Charge Control During Photomask CriticalDimension (CD) Metrology” written by David Joy, International SEMATECTechnology Transfer #3114452B-ENG(2004), the periphery of a specimen issurrounded by inert gas molecules at the time of electron beamirradiation. This method makes use of such an action that the inert gasions ionized by the electron beam neutralize the accumulated electriccharges of the specimen. In this method, the accumulated electriccharges themselves can be removed. In this method, however, thedetection efficiency of the secondary electrons much depends on thepressure of the inert gas around the specimen. Therefore in order toobtain a stable image, a fine control on the pressure of the gas isnecessary, which is a big problem in installation.

An object of the invention is to prevent from an image drift caused byan electrically charged specimen when observing the specimen includingan insulating material, in a charged particle beam system.

SUMMARY OF THE INVENTION

According to the invention, the charged particle beam irradiation systemhas a deflector for deflecting a particle beam, an objective lens forconverging the irradiating particle beam on the specimen, and asecondary electron signal detector for detecting a secondary electronsignal released from the specimen, hence to create an image according tothe signal from the secondary electron signal detector.

According to the invention, a first scan is performed in a predeterminedscanning direction on scanning line and a predetermined sequentialdirection of scanning lines and a second scan is performed in a scanningdirection different from the predetermined scanning direction and asequential direction different from the predetermined sequentialdirection.

An image may be created by repeating the process of executing the secondscan after executing the first scan and by requiring the arithmeticaverage of the frames obtained by the second scans. An image may becreated by averaging arithmetically at least one frame obtained by thefirst scan and at least one frame obtained by the second scan.

According to the invention, it is possible to prevent from an imagedrift caused by charge of a specimen when observing the specimenincluding an insulating material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a constitutional example of a charged particlebeam irradiation system according to the invention.

FIG. 2 is a view for use in describing a first example of a method forcreating an image according to a scanning electron microscope of theinvention.

FIG. 3 is a view for use in describing a second example of the methodfor creating an image according to the scanning electron microscope ofthe invention.

FIG. 4 is a view for use in describing a third example of the method forcreating an image according to the scanning electron microscope of theinvention.

FIG. 5 is a view for use in describing the flow of the processing in thethird example of the method for creating an image according to thescanning electron microscope of the invention.

DESCRIPTION OF REFERENCE NUMERALS

-   101: specimen-   103: target area-   103 a, 103 b, 103 c: frame (inverse)-   104: scanning direction (inverse) on scanning line-   105 a, 105 b: scanning direction (normal) on scanning line-   106 a, 106 b, 106 c: frame (normal)-   107: sequential direction (inverse) of scanning lines-   108: sequential direction (normal)-   109: scanning area-   110: asymmetry (inverse) in electric charge distribution-   111: asymmetry (normal) in electric charge distribution

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, an example of a charged particle beamirradiation system according to the invention will be described. In thisexample, the charged particle beam irradiation system is a scanningelectron microscope. The scanning electron microscope in the example hasan electron source cathode 205, a first anode 206, a second anode 207, afirst converging lens 208, an aperture plate 209, a second converginglens 210, an orthogonal field generator 214, a scanning coil 212, anobjective lens 211 and a specimen support 203. The specimen support 203is arranged in a specimen chamber 201. The specimen chamber 201 is keptin a high vacuum state of about 1×10⁻⁴ Pa by a vacuum exhaust system222.

The scanning electron microscope further has power sources 217, 218,219, and 220 for supplying voltage to be applied to the electron sourcecathode 205, the first anode 206, the second anode 207, the firstconverging lens 208, the second converging lens 210, and the objectivelens 211. The scanning electron microscope further has a scanning signalpower source 221 for supplying saw-tooth current to the scanning coil212.

The scanning electron microscope has a secondary electron signaldetector 215, a signal amplifier 216, a controller 223, a drawing device224, an image display device 225, an image storing device 226, and animage processor 227.

A method of obtaining an image of a specimen by the scanning electronmicroscope will be described in brief. First, a specimen 202 is arrangedon the specimen support 203 in the specimen chamber 201. The electronbeam 204 discharged from the electron source cathode 205 is acceleratedby the first anode 206 and the second anode 207, converged by the firstconverging lens 208, passing through the aperture plate 209, convergedby the second converging lens 210, and deflected by the scanning coil212. The electron beam 204 is further converged by the objective lens211, into a convergent beam having a minute cross sectional diameter,and arrives at the surface of the specimen 202.

Although the scanning mechanism of the example uses a magnetic fieldmethod using a magnetic field generated by the scanning coil, it canalso use an electric field method using an electric field generated byapplying voltage to the opposite electrodes. The secondary electronsignal (in this case, the secondary electron) 213 generated from thespecimen 202 is drawn by the voltage applied between the specimen 202and the objective lens 211 and proceeds beyond the objective lens 211.The secondary electron signal 213 is separated from the primary electronbeam 204 by the orthogonal field generator 214 and arrives at thesecondary electron signal detector 215. The secondary electron signal213 is converted into electric signal by the secondary electron signaldetector 215, amplified by the signal amplifier 216, and sent to thedrawing device 224. The drawing device 224 converts the secondaryelectron signal 213 into image signal and transfers it to the imagedisplay device 225. Thus, the specimen image is displayed by the imagedisplay device 225. The image signal is further transferred to the imagestoring device 226. The image processor 227 performs the averagingprocessing, detection of a specific shape, detection of an image shift,and measurement of a specific shape dimension on the image informationtaken in by the image storing device 226. Namely, the image processor227 performs the image processing described with reference to FIG. 2 toFIG. 5. All these elements are sequentially controlled by the controller223.

A first example of a method for creating an image according to ascanning electron microscope of the invention will be described withreference to FIG. 2. When observing a target area 103 on a specimen 101,generally, a “view search” for searching the target area 103 is firstperformed. In the view search, a relatively wide area 102 including thetarget area 103 is observed at a comparatively low magnification.Namely, scan is performed in the normal scanning direction 105 a, toobtain an image of the wide area 102. At this time, the positiveelectric charges produced by the discharge of the secondary electronsare accumulated in the wide area 102. After the target area 103 is foundin the wide area 102, the target area 103 is observed at a comparativelyhigh magnification.

In the conventional technique, when observing the target area 103, ascan is performed in a scanning direction 105 b, and the resultantframes 106 a, 106 b, and 106 c are arithmetically averaged, hence to getan image of the target area 103. At this time, since the scanningdirection 105 b on scanning line and the sequential direction 108 ofscanning lines are always constant, there occurs asymmetry 111 in thedistribution of the electric charges varying periodically, in thescanning area 109 for observing the target area 103. The asymmetry 111in the distribution of the electric charges creates an asymmetricelectric field above the scanning area 109 and deflects the incidentelectron beam. This is one of the reasons for causing an image drift.

According to the invention, when observing the target area 103, apreliminary scan is first performed and then a main scan is performed.At first, a first preliminary scan is performed. The preliminary scanand the main scan may be performed in the same number of times.

The preliminary scan is performed in an inverse scanning direction 104to the scanning direction 105 b of the main scan and in an inversesequential direction 107 to the sequential direction 108 of the mainscan. According to this, a first preliminary frame 103 a is obtained.Next, a first main scan is performed. The main scan is performed in thesame way as the conventional technique; in the normal scanning direction105 b on scanning line and in the normal sequential direction 108 ofscanning lines. Thus, a first main frame 106 a can be obtained. Next, asecond preliminary scan is performed to get a second preliminary frame103 b. A second main scan is performed to get a second main frame 106 b.Similarly, a third preliminary scan is performed to get a thirdpreliminary frame 103 c. A third main scan is performed to get a thirdmain frame 106 c. Thus obtained main frames 106 a, 106 b, and 106 c areaveraged arithmetically, hence to obtain an image.

The preliminary scan produces an asymmetry 110 in the distribution ofelectric charges periodically changing, in the scanning area 109. Themain scan after the preliminary scan produces an asymmetry 111 in thedistribution of electric charges periodically changing, in the scanningarea 109. However, the direction of the asymmetry 110 in thedistribution of electric charges caused by the preliminary scan isopposite to the direction of the asymmetry 111 in the distribution ofelectric charges caused by the main scan. Therefore, two asymmetries 110and 111 are cancelled out each other. When the frames 106 a, 106 b, and106 c obtained by the main scan performed after the preliminary scan areaveraged arithmetically, since the two asymmetries 110 and 111 arecancelled out, there occurs no image drift. According to this, it ispossible to eliminate an image drift caused by the charge of thespecimen. The direction of the asymmetries 110 and 111 in thedistribution of electric charges means the direction of the electricfield vector.

When the specimen is an insulator, the attenuation constant of theaccumulated electric charges is the same order as the frame formationtime. Therefore by overlapping the scans of the opposite directions, theasymmetries of the accumulated electric charges can be cancelled out.The scanning direction can be changed just by reversing the polarity ofthe saw-tooth current applied to the scanning coil 212 in FIG. 1.Namely, it is possible just by a change in the control procedureinstalled in the controller 223 without adding anything to the originalstructure of SEM shown in FIG. 1. The invention is to provide a devicefor cutting down the image drift occurring at the time of observing aninsulator, at a very low cost.

A second example of the method for creating an image according to thescanning electron microscope of the invention will be described withreference to FIG. 3. A first scan is performed in the inverse scanningdirection 302 and in the inverse sequential direction 308, to obtain afirst group of frames 301 a and 301 b. Next, a second scan is performedin the normal scanning direction 304 and in the normal sequentialdirection 309, to obtain a second group of frames 303 a, 303 b, and 303c. A third scan is performed in the inverse scanning direction 302 andin the inverse sequential direction 308, to obtain a third group offrames 301 c and 301 d. A fourth scan is performed in the normalscanning direction 304 and in the normal sequential direction 309, toobtain a fourth group of frames 303 d, 303 e, and 303 f.

In this example, all the frames 301 a, 301 b, 303 a, 303 b, 303 c, 301c, 301 d, 303 d, 303 e, and 303 f are arithmetically averaged, hence toobtain an image. The scan in the normal scanning direction 304 and inthe normal sequential direction 309 may be performed the same number oftimes as the scan in the inverse scanning direction 302 and in theinverse sequential direction 308.

The asymmetry 306 in the distribution of the electric charges producedin the scanning area 305 by the first and third scans is opposite indirection to the asymmetry 307 in the distribution of the electriccharges produced in the scanning area 305 by the second and fourthscans. Accordingly, the asymmetry 306 in the distribution of theelectric charges produced by the first and the third scans is cancelledout by the asymmetry 307 in the distribution of the electric chargesproduced by the second and the fourth scans. Therefore, any image driftdoes not occur.

The example of FIG. 3 is effective especially in the path where theaccumulated electric charges on the specimen are diffused, for example,when there exists a film of conductive material with high resistance andthe like and the attenuation constant of the accumulated electriccharges is small. Also in the example, the intervals of inserting theinverse frames and the number of the frames may be set previously in thecontroller 223 of FIG. 1, which can be realized just by changing thecontrol procedure.

A third example of a method for creating an image according to thescanning electron microscope of the invention will be described withreference to FIG. 4. In the example, a plurality of scans are performedin the normal scanning direction 402 and in the normal sequentialdirection 403, to obtain frames 401 a, 401 b, and 401 c. The scans areperformed at predetermined intervals of time. Therefore, the positionsof the image element 405 included in the respective frames 401 a, 401 b,and 401 c are shifted. Therefore a shift amount between the imagesincluded in the frames 401 a, 401 b, and 401 c is detected. This is theimage drift amount. The method for detecting the shift amount betweenthe images is known. For example, refer to John C. Russ:Computer-Assisted Microscopy; Plenum Publishing Corp. New York 1990: pp45-46. When the image drift amount exceeds a predetermined value, amethod shown in FIG. 2 and FIG. 3 is executed.

FIG. 5 is an example of algorithm for executing the third example of themethod for creating an image according to the scanning electronmicroscope of the invention. At first in Step S1, it is checked whetherthe image drift amount, in other words, the shift amount of the imagesis equal to or more than a predetermined threshold. When the shiftamount of the images is equal to or more than a predetermined threshold,the processing proceeds to Step S2, where the scan in the inversescanning direction on scanning line and in the inverse sequentialdirection of scanning lines is performed, as shown in FIG. 2 and FIG. 3.When the shift amount of the images is less than the predeterminedthreshold, the processing proceeds to Step S3, where the image creatingprocessing is performed without inversing the scanning direction and thesequential direction.

When the image magnification is, for example, one hundred thousandtimes, the image shift amount is about one nanometer/second in thethreshold used in Step S1.

In the example, the scan is performed inverting both the scanningdirection and sequential direction at 180 degree. However, either thescanning direction or the sequential direction may be inverted. Forexample, when the image drift has a circular motion on the screen,inversion only in the scanning direction is effective. The conversionangle may be any angle other than 180 degree. Further, a plurality offrames at different angles can be sequentially inserted and a methodmost suitable to a specimen can be searched for.

As set forth hereinabove, although the example of the invention has beendescribed, the invention is not restricted to the examples but it iseasily understood for those who skilled in the art that variousmodifications can be made within the scope of the invention disclosed inthe appended claims.

Although in the example, the scanning electron microscope has beenmainly used as an example, the invention can be applied to anirradiation device using charged particles other than the electron beam,for example, using kalium ion. Since it can be widely applied to ananalysis for an insulator, it can be applied especially to anobservation and an inspection device of a liquid crystal opticalelement, a surface acoustic wave device, and a photomask forsemiconductor industry.

1. A charged particle beam irradiation system having a deflector fordeflecting a particle beam, an objective lens for converging anirradiating particle beam on a specimen, and a secondary electron signaldetector for detecting a secondary electron signal released from thespecimen, to create an image according to the signal from the secondaryelectron signal detector, wherein a scan by the deflector using theparticle beam includes a first scan in a predetermined scanningdirection on scanning line and in a predetermined sequential directionof scanning lines and a second scan in a scanning direction differentfrom the predetermined scanning direction and in a sequential directiondifferent from the predetermined sequential direction.
 2. The chargedparticle beam irradiation system according to claim 1, wherein a processof executing the second scan after executing the first scan is repeatedand frames obtained by the second scans are arithmetically averaged,hence to create an image.
 3. The charged particle beam irradiationsystem according to claim 1, wherein at least one frame obtained by thefirst scan and at least one frame obtained by the second scan arearithmetically averaged, hence to create an image.
 4. The chargedparticle beam irradiation system according to claim 1, wherein thescanning direction on scanning line and the sequential direction ofscanning lines in the second scan are opposite to the scanning directionon scanning line and the sequential direction of scanning lines in thefirst scan respectively.
 5. The charged particle beam irradiation systemaccording to claim 1, wherein the first scan and the second scan arerespectively performed the same number of times.
 6. The charged particlebeam irradiation system according to claim 1, wherein when an imagedrift is less than a predetermined threshold, only the first scan isperformed and when the image drift exceeds the predetermined threshold,the first scan and the second scan are performed.
 7. An electron beamscanning image creating method comprising: deflecting an electron beamon a specimen, converging an irradiating electron beam on the specimen,detecting a secondary electron signal released from the specimen, andcreating an image from the secondary electron signal, wherein thedeflecting scan includes a first scan in a predetermined scanningdirection on scanning line and in a predetermined sequential directionof scanning lines and a second scan in an inverse scanning direction tothe predetermined scanning direction and in an inverse sequentialdirection to the predetermined sequential direction.
 8. The electronbeam scanning image creating method according to claim 7, wherein aprocess of executing the second scan after executing the first scan isrepeated and frames obtained by the second scans are arithmeticallyaveraged, hence to create an image.
 9. The electron beam scanning imagecreating method according to claim 7, wherein at least one frameobtained by the first scan and at least one frame obtained by the secondscan are arithmetically averaged, hence to create an image.
 10. Theelectron beam scanning image creating method according to claim 7,wherein the first scan and the second scan are respectively performedthe same number of times.
 11. The electron beam scanning image creatingmethod according to claim 7, wherein when an image drift is less than apredetermined threshold, only the first scan is performed and when theimage drift exceeds the predetermined threshold, the first scan and thesecond scan are performed.
 12. A scanning electron microscope having adeflector for deflecting an electron beam, an objective lens forconverging an irradiating electron beam on a specimen, a secondaryelectron signal detector for detecting a secondary electron signalreleased from the specimen, and an image processor for processing animage signal created according to the signal from the secondary electronsignal detector, wherein a scan by the deflector using the electron beamincludes a first scan in a predetermined scanning direction on scanningline and in a predetermined sequential direction of scanning lines and asecond scan in an inverse scanning direction to the predeterminedscanning direction and in an inverse sequential direction to thepredetermined sequential direction.
 13. The scanning electron microscopeaccording to claim 12, wherein executing the second scan after executingthe first scan is repeated and frames obtained by the second scans arearithmetically averaged, hence to create an image.
 14. The scanningelectron microscope according to claim 12, wherein at least one frameobtained by the first scan and at least one frame obtained by the secondscan are arithmetically averaged, hence to create an image.
 15. Thescanning electron microscope according to claim 12, wherein when animage drift is less than a predetermined threshold, only the first scanis performed and when the image drift exceeds the predeterminedthreshold, the first scan and the second scan are performed.